This invention relates to intrusion detection systems in general and more particularly to such systems employing a continuous loop of sensing devices and apparatus for assuring proper operating conditions of said loop.
The prior art is replete with a number of intrusion detection systems which employ a plurality of sensors in a closed loop. Each of the sensors in the loop operates to monitor a particular location or a particular area such as a window, door or other area. Essentially, the sensors consist of closed contacts which indicate that the premises being monitored are secured. As soon as a door or window is opened or some other intrusion occurs on the monitored area or zones, the sensor operates to open, thus providing an opened circuit.
Since a plurality of sensors are wired in a loop, an open circuit indicates the presence of an alarm. In any event, modern intrusion detection systems employ threshold monitoring circuits. Such circuitry exists to determine that there is a valid alarm condition rather than a spurious or transient condition. In any event, such systems experience operating changes due to aging, temperature and other environmental conditions. These changes can result in false alarms when the system is armed or operative to monitor a zone for intrusion. In spite of many advances made in security systems, the present invention operates to monitor the resistance of the loop both in an armed and disarmed mode. In this manner by adequately monitoring the resistance of the loop, the system logic can be sure that the loop is operating correctly and there will be no problem when the system is switched from an armed to a disarmed condition or vice versa.
It is a objective of many intrusion detection systems to provide an open circuit indication to the control panel when the system is disarmed and not operating to monitor a given zone or a plurality of zones. When the system is armed, the loop presents a finite impedance to the control panel which therefore can make an accurate determination as to whether or not a zone has been violated or intruded upon. It is, of course, understood that based on modern components and based on the sensing devices utilized such as switches and so on, the resistance of such devices changes over periods of time. Hence due to corrosion, temperature, humidity and other environmental changes, the absolute resistance of the loop can change on a daily basis and actually change substantially on a long-term basis.
In a loop which has been operating for long periods of time one can experience extreme changes in resistance such that the entire monitoring system becomes extremely unreliable and thereby is capable of indicating an alarm condition when no such condition exists.
It is an object of the present invention to monitor an intrusion detection system loop to determine whether or not the loop will be operating reliably between an armed and disarmed condition.
It is a further object of the present invention to provide a reliable detection system capable of operating with an intrusion detection loop to determine whether or not the loop has proper resistance when operated from a disarmed state to an armed state. Essentially, as one can ascertain, the prior art is replete with a number of devices which operate to monitor the resistance or continuity of a particular line or circuit.
For example, reference is made to U.S. Pat. No. 3,551,797 issued on Dec. 29, 1970 to J. D. Holder et al and entitled AUDIO OUTPUT DEVICE FOR TESTING LEAKAGE AND CONTINUITY OF THE CIRCUIT. This patent discloses a leakage and continuity circuit which includes two monitors for testing an attached line. There are two terminals which are used for continuity to detect a break in the lines. These terminals are used for the leakage test and namely, to detect the particular current flow in the line.
U.S. Pat. No. 3,284,707 issued on Nov. 8, 1966 to H. H. Clinton and entitled CIRCUIT CONTINUITY AND RESISTANCE TESTER HAVING AN AUDIBLE OUTPUT SIGNAL. This patent discloses a circuit for testing an attached circuit in a high or low resistance mode based on a position of a high/low switch.
U.S. Pat. No. 4,594,542 issued Jun. 10, 1986 to F. H. Fish and is entitled SOLID STATE HIGH/LOW RESISTANCE MONITOR. This patent shows a resistance monitoring circuit having high and low selectable ratios which are implemented by means of a switch.
In any event, as indicated, there are many patents which rely on two-mode testing arrangements. However, none of these patents are adaptable or relate to intrusion detection systems nor do any of the patents operate to solve the above-noted problems which are substantial problems in the reliable operation of intrusion detections systems.
Thus as will be explained, it is a primary object of the present invention to provide a system and method for verifying automatic line integrity and employed as a diagnostic for indicating whether or not an intrusion detection loop is operating under proper conditions and will operate to reliably indicate proper alarm condition.